FLOOD03848 - Laserfiche WebLink

Home Browse Search

I I I I I I I I ] ~ I I ~ I I I I I I I 31 38 1. An increase in bed width primarily due to the reduction in wash load. Part of this wider bed would be covered with pioneer vegetation. The cause of the increase 1s that, where river bank erosion occurs, old floodplain land 1s destroyed. This land 1s not replaced by a buildup of new floodplain land 1n other places. There is very little sediment load with which new bars and banks can be constructed. Thus, as the river moves laterally, its width increases. 2. A decrease in bankfull depth due mainly to a decrease in wash load and to a lesser extent 1n the decrease 1n flood peaks. The cause is the same as above. As the river roves laterally. its newly- built banks ue lower due to the reduction in wash load. 3. A decrease in sinuosity caused by the great decrease in wash load. As the valley floor slope will not change until the entire flood. plain has been reworked by the river, the intermediate.term response would be an increase in riverbed slope if no degradation occurred. 4. An increase in meander wave length due to the decrease in sinuosity. 5. A decrease in riverbed slope due mainly to the reduction in the supply of bed load to the river system. This decrease in slope is opposite to and greater than that caused by the decrease in sinuosity. 6. An increase in the size of materials making up the active part of the riverbed. Generally degradation causes odditional bank caving in areas where bank erosion was ~ccurring before. These failing banks are normally on the outside of bends. When the pool in a bend is deepened by degr~dation the bank height becomes greater so it is more likely to fail by slldin~. Conse- quently, it is anticipated that there would be more bank erosion wlth degradation than there is now. Degradation wiTT narrow the low-flow channel. If there is a supply of moisture for the exposed bars adjacent to the low-flow channel .during the growing season, these wlll become vegetated with pioneer specles~- usually willow and cottonwood. Vegetation can have a profound effect on river morphology if there is a supply of sediment to build bars into ~slands and floodplain land. At the present time, the lack of fine sediment ln the South Platte River in the Metropolitan Denver area precludes rapid b ui1dup of bars. In the present regime, vegetation on bars remains low and sparse. Floods are capable of scouring (stripping off leaves and branches) or destroying this vegetation. Encroachment of vegetation does not appear to be a problem in the South Platte River. Large influxes of sediment from new development within the metropolitan area should have only a local effect on the growth of the bars. Empirical relations between river slope and meander pattern indicate that the unchanneled reaches of the South Platte River in Metropolitan De~ver are near the threshold between meandering and braided. ~he implication.'s that any change in the supply of water would cause the rlver to change ,ts pattern drastically. With the anticipated decrease i~ flood peaks, the form should move more tOwards meandering. Without a plentlful supply of sediment, however, the ri~er cannot change rapidly or drastically from braided to neandering. in bankfull depth is due to degradation caused by narrowing the channel and the other part is the result of backfilling on the floodplain. Changes in sinuosity and meander ~ave length ~111 be due more to the effects of man-made river bank stabilization than to natural causes. 4. Because of the composition of the materials underlying the South Platte River, the decrease in riverbed slope will be arrested quickly in many reaches by an armor coat of gravel or cobbles. In the channelized and leveed sections of the South Platte River in Metropol1tan Denver, the river is no longer free tomove.laterally . It is presumed that ony bank caving in this reach will be repalred ifmlediately as the value of the land adjacent to the river is very high. Future responses will be limited to changes in bed slope caused by degradation. As many drop structures now exist to control and localize degradation, the amount of degradation in these reaches will be less than in the others. Furthermore, the river has already degraded to bed rock in many locations so future erosion of this hard bed will be at a very slow rate. 3. These changes are modified for three reasons. Firstly, when a reach of this river becomes wide, vegetation becomes established on its gravel and sand bars. With an adequate supply of fine sediment, the bars grow in height, the vegetation succeeds to different types, and new high banks are created. The evidence is that the lack of wash load in the South Platte River in Metropolitan Denver results in a very slow buildup of bars. Thus the vegetation on the bars remains only the pioneer species. Very wide bends of the river in Adams County have thin patches of willows over a large portion of the point bars. The surface of the bars are mostly bed-load materials--sand and gravel. Secondly, landowners will not allow the river to sweep across the floodplain destroying their valuable assets. Thirdly, the landowners encroach on the river channel by backfilling marginal riverbed areas to increase thei r' acreage of f100dplain land. -Consequently, because the river will notbecompletelyfreetoc hange, the long-term anticipated responses to the reduction in sediment load and flood peaks are more likely to be as follows. 1. ;. slight increase in bed width and decrease in bankfull depth where the lateral migration of the channel is arrested but no narrowing or levees are constructed. 2. A great reduction in bed width and large increase in bankfull depth if the lateral migration of the channel is arrested and the channel is narrowed by filling in one or both sides. Part of the increase